VARIATION IN SEX EXPRESSION IN CANADA YEW (TAXUS CANADENSIS)

Sex expression was measured in several Canada yew (Taxus canadensis Marsh.) populations of the Apostle Islands of Wisconsin and southeastern Minnesota to determine the extent of variation within and among populations. Sex expression was recorded qualitatively (monoecious, male, or female) and quantitatively (by male to female strobilus ratios or standardized phenotypic gender). No discernible trends in differences in sex expression among populations or habitats were recorded. Trends in sex expression of individuals within populations were complex. Small yews tended to be male or, if monoecious, had female-biased strobilus ratios. Large yews were monoecious but had male-biased strobilus ratios. Phenotypic gender, recorded as relative maleness, however, was negatively, but weakly, correlated with plant size. Gender distribution in four of five populations was bimodal, suggesting that cosexual populations consist of male and female morphs. Strobilus ratios of individuals in Apostle Island populations showed significant annual variation, but gender for these same plants was significantly correlated from year to year. Annual adjustments in gender were most pronounced in small yews. The results indicate that relative investment in male and female reproductive structures by Canada yew individuals is responsive to environmental variation, but sex expression also has a proximate genetic component.     

The influence of deer browsing on the reproductive biology of Canada yew (Taxus canadensis marsh.)

Summary Canada yew (Taxus canadensis) populations currently browsed by white-tailed deer (Odocoileus virginianus) or browsed by deer in the past had significantly lower production of male strobili, female strobili, and seeds than unbrowsed yew populations. Exclosure studies showed that protected yews produced significantly more male and female strobili than unprotected yews, but only after several years of protection. Seed production did not respond as readily to protection from deer perhaps because of reduced pollination levels in browsed yew populations. Previously unbrowsed yews were clipped at different levels of removal of available browse (control (no removal), 25%, 50%, 75%, and 100% removal) to simulate deer browsing. Reduction in male strobilus production was linearly related to clipping intensity in three years of observation. Female strobilus production was significantly reduced only at the 100% level of removal. Intermediate levels of clipping may have even stimulated production of female strobili. Analysis of covariance, with previous year's branch production as the covariate, showed no significant effect of clipping on male strobilus production except in the 100% removal group. Female strobilus production showed no such covariance with branch production. Effects of clipping on seed production could not be reliably assessed in 1984 and 1985 due to low seed production. Seed production in 1986 was significantly reduced only in the 100% removal group. Field observations of deer browsing of Canada yew indicate that 100% levels of removal are typical of natural levels of browsing.     

Indirect interactions between browsers and seed predators affect the seed bank dynamics of a chaparral shrub

Interactions between herbivores and seed predators may have long-term consequences for plant populations that rely on persistent seed banks for recovery after unpredictable fires. We assessed the effects of browsing by deer and seed predation by rodents, ants and birds on the densities of seeds entering the seed bank of Ceanothus cuneatus var. rigidus, a maritime chaparral shrub in coastal California. Ceanothus produced many more seeds when protected from browsers in long-term experimental exclosures than did browsed plants, but the seed densities in the soil beneath browsed and unbrowsed Ceanothus were the same at the start of an intensive one-year study. The density of seeds in the soil initially increased in both treatments following summer seed drop: while densities returned to pre-drop levels within a few weeks under browsed plants, soil seed densities remained high for 5–8 months beneath unbrowsed plants. Rodent abundance (especially deer mice) was higher near unbrowsed plants than >30 m away, and rodents removed Ceanothus seeds from dishes in the experimental plots. At least in the short term, rodent density and rates of seed removal were inversely related to the intensity of browsing. Our data have management implications for maintaining viable Ceanothus populations by regulating the intensity of browsing and the timing, intensity and frequency of fires.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Long-term impacts of goat browsing on bush-clump dynamics in a semi-arid subtropical savanna

The effects of 16 years of continuous browsing by goats in a South African savanna at stocking rates intended for bush control were compared with plots unbrowsed for the same period of time. Differences in bush-clump density, structure and species composition were recorded. Bush-clump density did not differ between browsed and unbrowsed plots. Within individual bush-clumps, browsing was shown to impact more on structure than species composition, with smaller, shorter bush-clumps, containing fewer species but much greater stem-densities. Although species presence/absence was little affected by browsing, many species showed differences in abundance, growth and location within browsed and unbrowsed bush-clumps. Species reduced in abundance in browsed plots included Cussonia spicata, Ehretia rigida, Grewia occidentalis, Jasminum angulare and Senecio linifolius. Several species showed reduced growth in browsed plots, particularly those located at bush-clump edges. The relatively unpreferred Aloe ferox was a notable exception. Although browsing had little effect on the composition of the main clump founding species, emergents or late arrivals, there were twice as many single plants in browsed plots and emergence of several species was restricted to the middle of bush-clumps. Comparison of our findings with aerial photographic evidence and other literature suggest that browsing alone is unlikely to significantly reduce scrub cover, although it can clearly control further expansion. Combinations of fire and browsing, rather than one factor alone, are considered likely to act fastest and most effectively to significantly reduce or remove scrub cover altogether.     

Facultative sex ratio shifts by a herbivorous insect in response to variation in host plant quality

Summary We tested predictions of sex allocation theory with a series of field experiments on sex allocation in an herbivorous, haplodiploid, sawfly, Euura lasiolepis. Our experiments demonstrated the following points. 1) Adult females allocated progeny sex in response to plant growth. 2) Population sex ratios varied in response to plant quality, being male-biased where plant growth was slow and female-biased where plant growth was rapid. 3) Family sex ratios varied in response to plant quality, being male-biased on slow-growing plants and female-biased on rapidly-growing plants. 4) Female fitness increased more rapidly as the result of developing on more rapidly-growing plants than male mass. We conclude from these results that there are unequal returns on investment in male and female progeny. This results in facultatively biased sawfly sex ratios as an adaptive response to variation in plant quality.     

Effects of competition,herbivory and substrate disturbance on growth and size structure in pin cherry (Prunus pensylvanica L.) seedlings

We examined separate and interactive effects of intraspecific competition, vertebrate browsing and substrate disturbance on the growth and size structure of pin cherry (Prunus pensylvanica L.) in the first two seasons of growth after clearcutting, in a hardwoods forest in New Hampshire, United States. Over the 15-month study period, 97.5% of 1801 individuals survived, and mean plant height increased from 4-fold at high density to 5-fold at low density. Relative height growth was significantly lower at higher plant densities in two of the three growth periods examined. Vertebrate browsers (moose and deer) significantly preferred taller plants. Browsed plants had higher relative height growth following browsing than unbrowsed plants (compensatory growth) at low and intermediate densities. The degree of compensation declined with density and compensation was not significant at the highest density level. At low and intermediate densities, plants browsed early in life regained height dominance through compensatory growth; they failed to regain dominance at high density. Because compensatory growth tended to offset the effects of size-selective browsing, there was no difference in the degree of size inequality between browsed and unbrowsed plots. However, size inequality increased with plant density. Substrate disturbance caused by logging had no significant effects on either relative height growth or size inequality. The slope of the relationship between relative height growth and initial height increased significantly with density and time, and was higher in unbrowsed than in browsed plots, suggesting that competition among plants was size-asymmetric. Despite the preference of browsers for large plants, there was a clear net growth advantage for plants of large initial size, when the effects of competition, browsing and compensatory growth were combined. The interactive effects of density and browsing demonstrate the importance of a multifactorial approach to the analysis of individual plant performance and population structure.     

Female mating behavior in the field cricket,Gryllus pennsylvanicus (Orthoptera: Gryllidae) at different operational sex ratios

The influence of operational sex ratio on the mating behavior of female field crickets,Gryllus pennsylvanicus, was investigated. Females were predicted to be more discriminating under conditions of high mate availability and show less selectivity when males were rare. Such selectivity was indicated in this study with the proportion of courtships leading to a mating changing with sex ratio. Females accepted almost 70% of all courtships at the female-biased sex ratio, but only about half of all courtships were successful at even or male-biased sex ratios. Females moved least at the female-biased sex ratio. There was also a trend for females to be guarded more under male-biased conditions. Female weight did not influence any of the behaviors examined.     

Large herbivores affect the stoichiometry of nutrients in a regenerating woodland ecosystem

Previous studies on the effects of herbivores on nutrient cycling have given little consideration to the relationship between soil nitrogen (N) and phosphorus (P) availability. Here we examined how browsing by red deer influences the relative availability of N and P in a regenerating woodland ecosystem. We found that removal of browsing by fencing for 14 years led to a shift from N toward P limitation of the dominant tree species Betula pubescens . This was evidenced by a significant increase in foliar N:P ratio of B. pubescens as a result of removal of browsing; mean N:P ratio of foliage from browsed areas was 13.2 suggesting that trees growing in browsed areas were N limited, whereas foliage from unbrowsed areas had a mean N:P ratio of 15.8, suggesting that these areas were more P limited. Further evidence of a shift toward P limitation in unbrowsed areas came from the finding that root uptake of labelled ³²P was significantly greater in roots collected from unbrowsed than browsed trees. Soil phosphatase activity did not significantly differ between browsed and unbrowsed areas. Our data indicate therefore that herbivores have the potential to significantly affect the stoichiometry of N and P in forest ecosystems.     

Gender plasticity in Sagittaria sagittifolia (Alismataceae), a monoecious aquatic species

 Aquatic plants are well known for their high degree of phenotypic plasticity in vegetative structures, particularly leaves. Less well understood is the extent to which their sexuality can be modified by environmental conditions. Here we investigate gender plasticity in the European clonal monoecious aquatic Sagittaria sagittifolia (Alismataceae) to determine how floral sex ratios may vary with plant size and inflorescence order. We sampled two populations from aquatic habitats in East Anglia, U.K. and measured a range of plant attributes including ramet size and the number of female and male flowers per inflorescence. The two populations exhibited similar patterns of phenotypic gender, despite contrasting patterns of total allocation to female and male flower number. Plants produced male-biased floral sex ratios but female flower number increased from the first to the second inflorescence whereas male flower number decreased. Size-dependent gender modification occurred in both populations, but the patterns of allocation to female flower production differed between the two populations. Our results are consistent with the view that monoecy is a sexual strategy that enables plants to adjust female and male allocation in response to changing environmental conditions. Received September 16, 2002; accepted October 23, 2002 Published online: March 20, 2003     

Patterns of change in secondary and tertiary sex ratios of the Terebrantian thrips,Frankliniella occidentalis

Adult sex ratios (tertiary sex ratios) of the haplo-diploid Terebrantian thrips species, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) were examined from several native and introduced hosts in an apple (Malus spp.) ecosystem. The ratios were variable and most were female-biased, ranging from 0 to 61% males. Populations in apple bud clusters during early bloom were female-biased (0–10% males). At later bloom states, populations were less female-biased in open blossom clusters (up to 33% males) compared with those in either unopened buds or older petalless blossoms (<11% males). Changes in tertiary sex ratio in all hosts were associated with changes in resource quality, suggesting a possible effect of host quality on sex allocation. In contrast, larval (secondary) sex ratios within apples were slightly male-biased (57%) with little change across apple bud reproductive stages or over time, indicating little to no differential sex allocation. Sex ratios of populations in flight were less female-biased than those found on plants in 62 of 65 comparisons, which suggests that males are more likely to be in flight relative to females. The evidence supports that differential dispersal and distribution of sexes toward various host qualities together with a shorter longevity of adult males determines the pattern of adult sex ratios rather than any significant differential sex allocation.     

Short-Term Effects of Cattle Browsing on Tree Sapling Growth in Mountain Wooded Pastures

Browsing by livestock has been identified as an important factor preventing tree regeneration in wooded pastures. Two field experiments were performed to investigate the effects of cattle browsing on tree sapling growth in a mountain-wooded pasture. Two size classes (ca. 12–17 cm and 41–59 cm) of each of 4 species (Picea abies, Abies alba, Acer pseudoplatanus and Fagus sylvatica) were exposed to zero, low and high grazing intensities. We measured the proportion of saplings browsed and the effect of browsing on growth. A mowing treatment within the zero grazing intensity treatment showed no significant effect on sapling growth. One percent of the large saplings but 25% of the small saplings escaped browsing. Saplings were better protected when surrounded by taller vegetation. The proportion of saplings browsed was not significantly different among species although evergreen tree saplings lost a larger proportion of biomass than deciduous species when browsed. Under grazing, average current-year shoot production and total aboveground biomass of all species were significantly reduced. Browsing effects tended to be smaller at the lower grazing intensity. Because the deciduous species were least reduced in aboveground growth, especially under the low grazing intensity, we conclude that at least in short-term, deciduous species are less affected by cattle browsing.     

Winter browsing on Alaska feltleaf willow twigs improves leaf nutritional value for snowshoe hares in summer

In boreal forests, browsing by mammals on winter-dormant twigs increases leaf nitrogen, leaf greenness, and leaf size. This suggests browsing reduces competition among meristems for mineral nutrients, and in particular, competition for nitrogen. Winter browsing also reduces the shoot carbohydrate reserves used by leaves to produce condensed tannin. These effects of winter browsing are predicted to improve the nutritional value of leaves for mammals because they increase the mass of digestible nitrogen in leaves. This hypothesis was tested using Alaska feltleaf willow and the snowshoe hare as the experimental system. Six in vivo indicators of leaf nutritional quality were used to compare leaves from winter-browsed plants with leaves from unbrowsed plants. The indicators used were dry matter intake, nitrogen intake, condensed tannin intake, dry matter digestibility, apparent digestibility of nitrogen and nitrogen retention. The results obtained were in agreement with the above hypothesis. In early summer, at the time snowshoe hares and other northern herbivores reproduce, hares fed leaves from browsed plants consumed more nitrogen, digested more of the nitrogen they consumed, and retained more of the nitrogen they digested than did hares fed leaves from unbrowsed plants. The high nitrogen content and low tannin content of leaves from browsed plants may explain this browsing caused increase in leaf nutritional value. How these positive effects of winter browsing on snowshoe hare nutrition at the time of reproduction might affect hare population dynamics are briefly discussed.     

Structural diversity of naturally regenerating Chinese yew (Taxus wallichiana var. mairei) populations in ex situ conservation

The Chinese yew (Taxus wallichiana var. mairei) is ranked in the first class of important wild endangered plants in China. Due to overexploitation, it now occurs scattered only in the forest undergrowth along the Yangtze River Valley. To improve the conservation management of the species, we applied structural indices to investigate the structural diversity of naturally regenerating yew populations that have established via ex situ conservation. The results show that most yews had larger non‐yew tree neighbors; these were 30–70% larger than their reference trees. Collectively, the average distances between the yews and the three nearest‐neighboring trees were short ( 3 m), suggesting that the yews face strong interspecific competition from neighbors. In these two forest stands, most of the pole‐sized yews were found beneath a single tall neighboring tree (height  10 m), and their growth was enhanced under a single neighboring tree but not under two, three or zero neighboring trees. Finally, we recommend simple silvicultural measures to reduce interspecific competition and improve the vitality of the yew population; specifically, the cutting or pruning of branches of large neighboring trees in tandem with the thinning of canopy trees growing next to the mother yews.     

Chemical and morphological variation of Mediterranean woody evergreen species: Do plants respond to ungulate browsing?

Abstract. Plant defense theories suggest that chemical or structural defences should be maximized when and where browsing is most likely to occur. We tested this hypothesis on four evergreen woody species growing in a Mediterranean area with a high density of ungulates. In this system, levels of browsing are more intense in the winter (due to the lack of annual plants) and young foliage is often preferred. Therefore we predicted that the chemical defences of these species, namely their phenolic content, would vary with leaf age, season and damage intensity. In addition, we tested whether ungulates preferentially selected species containing lower phenolic levels, and also whether browsing induced either chemical or morphological changes in damaged plants. Phenolic levels varied greatly between plant species; ungulates browsed preferentially on the species with the lowest phenolic levels. No difference in phenolic content was found between browsed and unbrowsed trees. Morphological changes in heavily browsed trees included an increase in shoot and leaf density and a net decrease in leaf size. We suggest that for Mediterranean plants, which have evolved under high browsing pressure from large mammals, the production of small leaves and dense shoots in response to browsing might decrease ungulate foraging efficiency and hence reduce the rate of further damage as effectively as high levels of chemical defence.     

Resource utilization and sex allocation in response to host size in two ectoparasitoid wasps on subcortical beetles

Patterns of host resource utilization and sex ratio manipulation in relation to host size were investigated for two solitary ectoparasitoid wasps,Atanycolus initiator andSpathius brevicaudis (Hymenoptera Braconidae). Both species parasitize subcortical beetles on the trunks of Japanese pine trees.A. initiator is on average 8 times larger in body weight and has an ovipositor that is 3.7 times longer than that ofS. brevicaudis. In both parasitoids, the size of emerging wasps was positively correlated with host size, but the host/wasp size regressions were linear for all three major host species inA. initiator, whereas inS. brevicaudis the regression was logarithmic for a relatively large host species. The sex ratios (proportion of males) of both parasitoids emerging from different host species decreased with increasing host size, but the overall sex ratio on each host species was male-biased inA. initiator, while female-biased inS. brevicaudis. How the proportion of host consumed changed in response to host size, differed between the two parasitoids for the same host species. In the field survey, the size and sex ratio of the emerging two parasitoids from a dead tree were closely related to host size. However, the spatial distribution of the two parasitoids depended on the bark thickness of the trunk. The data suggest that differences in the relative evaluation of host size and in ovipositor length may enable the coexistence of the two parasitoid wasps.     

Ungulate browsing affects subsequent insect feeding on a shared food plant,bilberry (Vaccinium myrtillus)

Browsing by ungulates may induce plant responses and affect subsequent plant food quality for other animals. Populations of many deer species have increased to unprecedented levels in Europe and North America. In Norway, population densities of red deer (Cervus elaphus) have increased over the past decades, but little is known about how increased deer browsing pressure may change the palatability of key food plants for other taxa in the boreal ecosystem. We conducted a cafeteria experiment to assess if long-term deer-browsing intensity affected the palatability of bilberry (Vaccinium myrtillus) leaves for leaf-eating larvae (mainly Lepidoptera). We found that leaf-eating insect larvae preferred bilberry leaves from the lightly browsed bilberry plants; the larvae consumed twice as much leaf biomass from the lightly browsed plants than from the unbrowsed and moderately browsed ones, and four times more than from highly browsed plants. Larvae never selected leaves from highly browsed plants as their first choice. Our study suggests that browsing-induced changes in the quality of shared food plants may be important in mediating indirect interactions between browsers of widely separated taxa. Whereas low levels of long-term red deer browsing increases the palatability of bilberry leaves for leaf-eating larvae, high browsing pressure reduces food consumption. Whether changes in palatability lead to changes in population densities of leaf-eating larvae remains to be studied, but any such adverse effects could have cascading ecological consequences for insectivorous birds and mammals.     

Sex-ratio distortion driven by migration loads

The significance of migration load in driving the evolution of recipient populations has long been documented in population genetics, but its effects have not been linked to the formation of biased sex ratios in natural populations. In this study, we develop a single-locus model to demonstrate how the migration load can shape the primary and secondary sex ratios in dioecious plants where sexual dimorphism is determined by the sex chromosomes (the XX-XY or similar systems). Our results show that migration load can generate an array of sex ratios (from the female- to male-biased primary/secondary sex ratios), depending on the selection systems at the gametophyte and sporophyte stages and on the sex ratio in the migrating seeds. Ovule abortion and the purging of maladaptive genes from the immigrating pollen at the gametophyte stage can alter the primary sex ratio and indirectly alter the secondary sex ratio. The presence of maladaptive sex-linked genes from the migrating pollen and seeds of males facilitates the outcome of the female-biased secondary sex ratios, while the presence of maladaptive sex-linked genes from the migrating seeds of females can lead to the male-biased secondary sex ratios. The detrimental effects of the Y-chromosome from the migrating pollen and seeds can enhance the formation of female-biased primary and secondary sex ratios. These theoretical predictions highlight an alternative approach to the existing sex-ratio theories for interpreting the formation of biased sex ratios in the populations that are subject to the impacts of maladaptive genes from immigrants.     

Patterns of sex ratio variation and genetic diversity in the dioecious forest perennial <Emphasis Type="Italic">Mercurialis perennis</Emphasis>

In small populations of plant species with separate sexes, it can be expected that besides the local environment also stochastic events influence population sex ratios. Biased sex ratios may in turn negatively affect genetic diversity due to increased genetic drift and, in clonal plants, due to reduced sexual reproductive output. Empirical evidence for these processes is scarce, however. We investigated the pattern of sex ratio variation and the distribution of genetic variation of the dioecious clonal forest herb Mercurialis perennis using AFLP markers. Analysis of molecular variance indicated a pronounced genetic structure. Overall within-population genetic diversity was moderate and local sex ratios were slightly male biased. The proportion of male to female plants in large populations slightly increased with increasing light penetration to the herb layer. Small populations, on the contrary, displayed high variability in sex ratios, unrelated to the local light environment. Genotypic diversity decreased with more male-biased sex ratios. We conclude that stochastic events related to small population size and the local forest environment, related to canopy closure, affect the proportion of female plants and indirectly influence local genotypic diversity, likely through the degree of sexual reproduction. This is one of the first studies to report a clear association between gender proportions and genetic diversity of a dioecious plant species in a fairly large survey.     

Sex ratio in populations of Silene otites in relation to vegetation cover,population size and fungal infection

Abstract. In contrast to populations of most dioecious Silene species (which usually are female-biased), populations of Silene otites have been frequently reported to be male-biased. We describe sex ratio variation in 34 natural S. otites populations in Central Germany in relation to vegetation cover, population size and fungal infection. The overall sex ratio was unbiased in 1994 and only slightly male-biased in 1995. Sex ratio varied among the populations from 26.6 % to 72.6 % females. The sex ratio of small populations varied strongly due to stochastic processes. Furthermore, we found that populations in habitats with high vegetation cover contained a higher percentage of females. Hermaphroditic plants, theoretically, could increase male bias as they only produce male or hermaphroditic offspring. Their frequency in the populations, however, was far too low to affect sex ratio. In 1994 12.1 % and in 1995 17.0 % of the plants were infected by the smut fungus Ustilago major. Disease incidence in the population was not related to sex ratio, suggesting equal susceptibility of males and females. The sex ratio of partially infected plants did not deviate from the population sex ratio, both under field conditions and in a greenhouse laboratory experiment. The results suggest that the frequently reported male bias in Silene otites populations is not a general pattern, but is mainly caused by environmental conditions.